Iron (Fe) is an essential nutrient for almost all organisms. Enteric bacteria, including Salmonella and commensal/pathogenic Escherichia coli, require micromolar concentrations of Fe for replication and colonization in the vertebrate host. As levels of “free” Fe in the host are low, these bacteria biosynthesize and export small-molecule metal-ion chelators called “siderophores,” which scavenge Fe from the host.